In recent years, significant upsizing of steel structures has led to a marked increase in the strength and the thickness of steel that is used. Thick steel plates having a plate thickness of 100 mm or more are usually manufactured by slabbing a large steel ingot produced by an ingot making method, and hot rolling the resultant slab. In this ingot making-slabbing process, densely segregated areas in hot tops and negatively segregated areas in ingot bottoms have to be discarded. This causes low yields, high production costs and long work periods.
In contrast, a process using a continuously cast slab as the material steel is free from such concerns. However, the fact that the thickness of a continuously cast slab is less than that of an ingot slab causes the rolling reduction to the product thickness to be low. In the production of thick steel plates having increased strength, alloying elements are added in large amounts to ensure desired characteristics. This results in the occurrence of center porosities ascribed to center segregation, and the upsizing of steels consequently encounters the problematic deterioration of internal quality.
To solve this problem, the following techniques have been proposed for the purpose of improving the characteristics of center segregation areas by compressing center porosities during the process in which continuously cast slabs are worked into ultrathick steel plates.
Tetsu to Hagane (Iron and Steel), Vol. 66 (1980), No. 2, pp. 201-210 describes a technique in which center porosities are compressed by increasing the rolling shape factor during the hot rolling of a continuously cast slab. Japanese Unexamined Patent Application Publication Nos. 55-114404 and 61-273201 describe techniques in which center porosities in a continuously cast slab are compressed by working the continuously cast slab with rolls or anvils during its production in the continuous casting machine.
Japanese Patent No. 3333619 describes a technique in which a continuously cast slab is worked into a thick steel plate with a cumulative reduction of not more than 70% such that the slab is forged before hot rolling to compress center porosities. Japanese Unexamined Patent Application Publication No. 2002-194431 describes a technique in which a continuously cast slab is worked into an ultrathick steel plate by forging and thick plate rolling with a total working reduction of 35 to 67%. In that process, the central area through the plate thickness of the steel is held at a temperature of 1200° C. or above for at least 20 hours before forging and the steel is forged with a reduction of not less than 16% to eliminate center porosities and also to decrease or remedy the center segregation zone, thereby improving temper brittleness resistance characteristics.
Japanese Unexamined Patent Application Publication No. 2000-263103 describes a technique in which a continuously cast slab is cross forged and then hot rolled to remedy center porosities and center segregation. Japanese Unexamined Patent Application Publication No. 2006-111918 describes a technique related to a method of manufacturing thick steel plates with a tensile strength of not less than 588 MPa in which a continuously cast slab is held at a temperature of 1200° C. or above for at least 20 hours, forged with a reduction of not less than 17%, subjected to thick plate rolling with a total reduction including the forging reduction of 23 to 50%, and quench hardened two times after the thick plate rolling, thereby eliminating center porosities and also decreasing or remedying the center segregation zone.
Japanese Unexamined Patent Application Publication No. 2010-106298 describes a technique related to a method of manufacturing thick steel plates with excellent weldability and ductility in the plate thickness direction wherein a continuously cast slab having a prescribed chemical composition is reheated to 1100° C. to 1350° C. and thereafter worked at not less than 1000° C. with a strain rate of 0.05 to 3/s and a cumulative working reduction of not less than 15%.
The technique described in Tetsu to Hagane (Iron and Steel), Vol. 66 (1980), No. 2, pp. 201-210 requires that steel plates be repeatedly rolled with a high rolling shape factor to achieve good internal quality. However, such rolling is beyond the upper limit of equipment specifications of rolling machines and, consequently, manufacturing constraints are encountered.
The techniques of JP '404 and JP '201 have a problem in that large capital investments are necessary for adaptation of continuous casting facilities, and also have uncertainty about the strength of steel plates obtained. The techniques of JP '619, JP '431, JP '103, JP '918 and JP '298 are effective to remedy center porosities and improve center segregation zones. However, the yield strength of steel plates obtained is less than 620 MPa. Thick steel plates with a yield strength of 620 MPa or above decrease their toughness due to the increase in strength. Further, thick steel plates are cooled at a lower rate in the central area through the plate thickness than in the other areas. It is necessary to increase the amounts of alloying elements that are added to ensure strength in such central regions. Such thick steel plates containing large amounts of alloying elements increase their deformation resistance and, consequently, center porosities are not sufficiently compressed and tend to remain after the working. Thus, there is a concern that the steel plates will exhibit insufficient elongation and toughness in the central area through the plate thickness. As discussed above, there are no established techniques which realize thick high-toughness high-strength steel plates having a yield strength of 620 MPa or above, and methods of manufacturing such steel plates with existing facilities.
It could therefore be helpful to provide thick high-toughness high-strength steel plates with a yield strength of 620 MPa or above that contain large amounts of alloying elements and still have excellent strength and toughness in the central area through the plate thickness, as well as to provide methods of manufacturing such steel plates. The plate thickness of interest is 100 mm or more.